The invention relates to recycling dry, difficult to hydrate, fines into a main product stream. More particularly, the invention relates to reprocessing fines generated in the production of fluid absorbent polymers and copolymers.
Water-swellable polymers and copolymers are well known for their use as absorbents for aqueous fluids in diapers, sanitary products and the like. Certain of these polymers, such as those prepared from monomers of acrylic acid or methacrylic acid or their respective alkali metal or ammonium salts, and lightly crosslinked with a crosslinking monomer having at least two polymerizable double bonds, exhibit exceptionally high capacity for adsorbing liquids and are further characterized as having good physical integrity in processing and use.
These polymers/copolymers are typically employed in a particulate form that promotes contact with the fluid to be adsorbed. Good physical integrity of the particulate is a key desirable quality because the polymer/copolymer fines often lack the ability to absorb fluids that the desired particulate product possesses. In addition, the presence of fines in some absorbent products may significantly adversely affect the overall absorbing performance of the product.
In the production of acrylic acid-based copolymers, a significant portion of fines material, that is, less than 100 mesh (149 micrometers), is typically generated from the process of manufacturing the absorbent product. These processes generally include drying the polymer or copolymer gel, followed by breaking up and grinding to a final acceptable particle size range. In the course of the process, 8-11 percent by weight of the final product may be fines.
Initially, users employed the entire dry product, including fines, in their absorbent products. It was soon discovered, however, that the inclusion of fines resulted in lower product performance. The difficulty occurs when the fine particles are contacted with an aqueous fluid. The contact results in a "gel blocking" phenomenon. Upon hydration of a tightly packed mass of fines, only the outside layer is wetted because the fines form such a dense polymeric network that neither capillary action nor diffusion will permit penetration of the fluid into uniform contact with the interior particles. The result is a substantially reduced overall capacity of the absorbent polymer to absorb and hold aqueous fluids.
An initial solution to the fines problem was simply to screen the fines from the product. The resulting fines were stored as off-specification product. Attempts at directly recycling the fines into the process were heretofor unsuccessful even where wetting of the dry fines was attempted. Early trials resulted in products that would not dry or only dry at uneconomical rates. The recycled fines also failed in producing a particulate that would not disintegrate in processing subsequent to drying (such as sizing, pneumatic conveying or screening, for example) necessary for forming commercial products.
Thus, it would be desirable to provide a process that recycles fines into a main product stream of polymer/copolymer such that the finished product absorbent capacity and particulate integrity are equivalent to the material normally produced of a desired particle size. Such a process should not add significantly to drying time.